A MOS (Metal Oxide Semiconductor) transistor is a field effect transistor including a gate having a MOS structure. Such a transistor is called MOS-FET. The MOS transistor has been used inmost of memories, microprocessors, and the like, because the MOS structure is suitable for forming an integrated circuit. Among these, a thin film transistor (herein after, also referred to as “TFT”) including a semiconductor thin film layer made of silicon and the like as an active layer has been used in various applications, for example, as a switching element in a liquid crystal display device.
For display devices such as a liquid crystal display device including such a TFT, further reduction in power consumption has been strongly needed, in addition to increase in the screen size and improvement in definition of image displays. Also for the TFT used in the display device, reduction in power consumption has been needed. In order to reduce the power consumption of the TFT, it is essential to form a gate insulating film to have a thinner thickness of about 70 nm or less, for example, thereby operating the TFT with a low threshold. A silicon oxide film excellent in interface characteristics is generally used as the gate insulating film. If the gate insulating film has a single layer structure of a silicon oxide film, the silicon oxide film is thinned and thereby the TFT can be operated with a low threshold. However, a breakdown voltage, that is, a transistor withstand voltage is reduced, and defects such as a leakage defect between the semiconductor layer and the gate electrode layer are easily caused. Accordingly, if the TFT having a gate insulating film consisting of a single silicon oxide film is used as a switching element in a liquid crystal display device, point defects in a panel initial condition, defects in device reliability are increased, which possibly leads to reduction in yield. For this problem, the temperature at which the silicon oxide film is formed is decreased, and thereby the coverage of the silicon oxide film can be improved. However, the decrease in the film formation temperature reduces a flat band voltage of the silicon oxide film and increases the threshold. Therefore, a gate insulating film which satisfies both of the coverage and the low threshold is hard to obtain.
Materials with a dielectric constant higher than a dielectric constant of silicon oxide, for example, silicon nitride, have been recently used for the gate insulating film. According to a gate insulating film having a single layer structure consisting of a silicon nitride film, a gate insulating film having a two-layer structure including stacked silicon oxide film and silicon nitride film, and the like, the film thickness needed to obtain a capacitance equivalent to a capacitance in the case where the gate insulating film having a single layer structure consisting of a silicon oxide film becomes large because of the difference in dielectric constant between the silicon oxide and the silicon nitride, and therefore, reduction in withstand voltage can be suppressed. Further, the transistor withstand voltage can be improved. For example, a TFT including a gate insulating film having a two-layer structure including stacked lower silicon nitride film (on the gate electrode side) and upper silicon oxide film (on the semiconductor layer side) is disclosed (for example, refer to Patent Document 1). However, according to this TFT, the silicon oxide has a large thickness of 120 nm or more. Therefore, it is difficult to simultaneously realize a low threshold operation of the TFT and production of a liquid crystal display device with low power consumption.
Accordingly, the TFT including a gate insulating film having a multilayer structure also has a room for improvement in that the low threshold operation becomes difficult if the transistor withstand voltage is increased by improvement in coverage.
[Patent Document 1]
Japanese Kokai Publication No. Hei-11-111991